1. Field of Invention
The present invention relates to a manufacturing method of micro-electro-mechanical system (MEMS) device, especially a manufacturing method including: first providing a structure layer to connect a substrate, and afterward providing a cap to cover the structure layer and a portion of the substrate such that a movable structure is not connected to the cap for improving sensing stability, and a MEMS device made thereby.
2. Description of Related Art
MEMS devices are often used for sensing motion or pressure, such as acceleration sensors, gyroscopes, altitude sensors, etc. AMEMS device includes a micro-electro-mechanical device in cooperation with an integrated circuit. There are three approaches to integrate a micro-electro-mechanical device and an integrated circuit: two-chip solution, CMOS-MEMS solution, and wafer-level integration solution.
FIG. 1 shows a MEMS device 10 according to the two-chip solution. A micro-electro-mechanical device chip 11 and an integrated IC chip 12 are separately packaged into chips and wire-bonded for signal transmission (as illustrated by the solid dots in figure) . The two-chip solution has an advantage that the manufacturing process is simple, because the micro-electro-mechanical device chip 11 and the integrated circuit chip 12 are separately manufactured and packaged; however, the two-chip solution has a drawback that the separate packages require related pads and pins which increase the cost and consume a larger area. Further, the two-chip solution has another drawback that the wire bond may generate parasitic effects (such as parasitic capacitances formed by the pads), resulting in noises, and the parasitic effects may further influence other electrical behaviors of the MEMS device 10. The problem is that the the parasitic capacitances and the change of these in operation can not be accurately predicted in the design stage for the micro-electro-mechanical device chip 11 and the integrated circuit 12, and can not be accurately compensated in manufacturing the chips; therefore, the sensing performance of this solution is less accurate.
FIG. 2 shows a MEMS device 20 according to the CMOS-MEMS solution. A micro-electro-mechanical device 21 and an integrated circuit 22 are manufactured on one semiconductor wafer but located at different positions. FIG. 2 shows an example that the micro-electro-mechanical device 21 and the integrated circuit 22 are located at two different regions on one same plane; there are other examples (not shown) wherein the micro-electro-mechanical device 21 and the integrated circuit 22 are processed in series. This solution has a drawback that the manufacturing process is more complicated because the micro-electro-mechanical device 21 and the integrated circuit 22 have different requirements. For example, the stack of metal interconnections and isolation layers in manufacturing the integrated circuit 22 may cause a structure distortion in the micro-electro-mechanical device 21 because of different thermal expansion coefficients of different layers, leading to non-ideality effect on signal distortion and large performance shift caused by temperature change. Because the micro-electro-mechanical device 21 and the integrated circuit are manufactured on a same wafer, the process of micro-electro-mechanical device 21 could limit the selection of the integrated circuit 22. For example, the integrated circuit 22 could be improved by selecting more advance technology, but the micro-electro-mechanical device 21 does not. Thus, this type of integration cannot provide the state of art performance due to the different requirements regarding the mechanical and electrical characteristics.
FIGS. 3 and 4 show a MEMS device 30 according to the wafer-level integration solution. The micro-electro-mechanical device and the integrated circuit are separately manufactured in different wafers and afterward bonded in wafer form, whereby a micro-electro-mechanical device 31 is stacked on an integrated circuit device 32, and the stack structure of the micro-electro-mechanical device 31 and the integrated circuit device 32 is singulated from the stacked wafers and packaged into a MEMS device chip. As shown in FIG. 3, the micro-electro-mechanical device 31 and the integrated circuit device 32 have respectively finished their own manufacturing processes, wherein the micro-electro-mechanical device 31 includes a movable structure 312, a cap 313, and a connection section 314 for connecting the movable structure 312 and the cap 313 (the location of the connection section 314 is for illustrative purpose and not limited to the location as shown in figure). The integrated circuit device 32 includes a substrate 321, a circuit layout (not shown), signal contacts 322 for connection to the micro-electro-mechanical device 31, and a bond pad 323 for external communication. If necessary, a chamber 324 can be formed to provide a working space for the motion of the micro-electro-mechanical device 31. In FIG. 4, the micro-electro-mechanical device 31 and the integrated circuit device 32 are bonded to each other, and the bond pad 323 can be exposed by etching the micro-electro-mechanical device 31 (related step not shown).
The prior art shown in FIGS. 3-4 has the following drawback. To manufacture the micro-electro-mechanical device 31 in the semi-finished state as shown, the movable structure 312 which is suspending needs to be connected to a fixed part, so the movable structure 312 must be connected to the cap 313. However, a pressure is laid on the cap 313 when the micro-electro-mechanical device 31 is bonded with the integrated circuit device 32, and the molding step in the packaging process and the mounting step of the MEMS device on a circuit board will also cause the cap 313 to be stressed. Because the cap 313 is connected to the movable structure 312, the stressed cap 313 will influence the structure and motion of the movable structure 312 to cause inaccuracy, and this inaccuracy cannot be predicted and compensated in the design stage or in the manufacturing process of the micro-electro-mechanical device 31 and the integrated circuit device 32.